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Introduction

This project involves building a simple device that monitors air quality using an Arduino Uno and an MQ135 gas sensor. The sensor detects harmful gases and pollutants in the air, while the Arduino processes the data and displays air quality levels using LEDs and a buzzer.

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Materials Required

Item	Quantity
Arduino Uno	1
MQ135 Air Quality Sensor	1
Breadboard	1
Jumper Wires	Several
Green LED	1
Yellow LED	1
Red LED	1
220 Ω Resistors	3
Buzzer	1
USB Cable	1
Laptop with Arduino IDE	1

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Step 1: Understand How the Sensor Works

The MQ135 sensor contains a sensitive material that changes its electrical resistance when exposed to gases such as:

• Carbon dioxide (CO₂)
• Ammonia (NH₃)
• Benzene
• Smoke
• Alcohol vapours

The Arduino reads these changes and determines the air quality level.

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Step 2: Assemble the Circuit

Connect the MQ135 Sensor

MQ135 Pin	Arduino Pin
VCC	5V
GND	GND
A0	A0

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Connect the LEDs

Green LED

• Positive leg → Pin 4 through 220 Ω resistor
• Negative leg → GND

Yellow LED

• Positive leg → Pin 5 through 220 Ω resistor
• Negative leg → GND

Red LED

• Positive leg → Pin 6 through 220 Ω resistor
• Negative leg → GND

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Connect the Buzzer

Buzzer Pin	Arduino Pin
Positive	Pin 7
Negative	GND

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Step 3: Build the Circuit on the Breadboard

Arrange the components neatly:

1. Place the Arduino beside the breadboard.
2. Insert the MQ135 sensor on the breadboard.
3. Place the three LEDs in a row.
4. Connect all grounds to the breadboard ground rail.
5. Use jumper wires to complete all connections.

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Step 4: Install Arduino Software

1. Download Arduino IDE.
2. Install it on your computer.
3. Connect the Arduino using the USB cable.
4. Select:
   • Board → Arduino Uno
   • Port → COM Port of your Arduino

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Step 5: Upload the Program

Copy and paste the following code into Arduino IDE.

int sensorPin = A0;
int greenLED = 4;
int yellowLED = 5;
int redLED = 6;
int buzzer = 7;

void setup()
{
  Serial.begin(9600);

  pinMode(greenLED, OUTPUT);
  pinMode(yellowLED, OUTPUT);
  pinMode(redLED, OUTPUT);
  pinMode(buzzer, OUTPUT);
}

void loop()
{
  int airValue = analogRead(sensorPin);

  Serial.print("Air Quality = ");
  Serial.println(airValue);

  if(airValue < 200)
  {
    digitalWrite(greenLED, HIGH);
    digitalWrite(yellowLED, LOW);
    digitalWrite(redLED, LOW);
    digitalWrite(buzzer, LOW);
  }

  else if(airValue < 400)
  {
    digitalWrite(greenLED, LOW);
    digitalWrite(yellowLED, HIGH);
    digitalWrite(redLED, LOW);
    digitalWrite(buzzer, LOW);
  }

  else
  {
    digitalWrite(greenLED, LOW);
    digitalWrite(yellowLED, LOW);
    digitalWrite(redLED, HIGH);
    digitalWrite(buzzer, HIGH);
  }

  delay(1000);
}

Click Verify and then Upload.

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Step 6: Test the Device

After uploading:

Clean Air

• Green LED ON
• Yellow LED OFF
• Red LED OFF
• Buzzer OFF

Moderate Pollution

• Yellow LED ON

Heavy Pollution

• Red LED ON
• Buzzer sounds

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Step 7: Conduct Experiments

Measure air quality in different locations:

Location	Sensor Reading	Air Quality
Classroom
Laboratory
Kitchen
Roadside
Near Vehicle Exhaust

Record observations and compare results.

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Step 8: Prepare Your Science Fair Display

Include:

Title

Smart Air Quality Monitoring System Using Arduino and MQ135 Sensor

Sections

1. Introduction
2. Problem Statement
3. Objectives
4. Materials
5. Methodology
6. Circuit Diagram
7. Results
8. Discussion
9. Conclusion
10. Recommendations

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Expected Results

Reading	Interpretation
0–200	Good Air
200–400	Moderate Air
400–600	Poor Air
Above 600	Dangerous Air

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Advanced KSEF Upgrades

To make the project more competitive:

1. Add an OLED display showing AQI.
2. Add a DHT22 sensor for temperature and humidity.
3. Use an ESP8266 Wi-Fi module to send data online.
4. Create a mobile phone monitoring app.
5. Power the system using a solar panel.
6. Store data on an SD card for long-term analysis.
7. Use AI to predict pollution trends.

Science Behind the Project

The MQ135 sensor converts chemical changes caused by pollutants into electrical signals. The Arduino interprets these signals and provides a real-time indication of air quality, helping people identify polluted environments and take appropriate action.

This project combines Environmental Science, Electronics, Programming, Data Analysis, and Public Health, making it an excellent science fair project for secondary school and KSEF competitions